In recent years, the efficiencies of automotive engines have increased significantly in order to conserve fuel and to comply with statutory and regulatory requirements on automotive fuel consumption. This increased efficiency has, in turn, led to more severe service requirements for the engine lubricants because the higher efficiencies have generally been accompanied by higher engine temperatures as well as higher bearing pressures concommitant upon the use of higher compression ratios. These increasingly severe service requirements have made it necessary for lubricant manufacturers to provide superior lubricants. Furthermore, it is expected that this trend will continue and that in the future even more severe service ratings will be established by engine manufacturers. At present, the API "SF" rating is currently employed for gasoline engines and this in itself represents a significant increase in the service requirements of lubricants; soon, however, it is expected that even more severe service ratings represented by the API "SG" will become current. Thus, there is a continuing need for lubricants with superior performance characteristics.
One of the performance characteristics which is of greatest significance is the viscosity index (VI). This represents the extent to which the viscosity of a lubricant varies with temperature. Lubricants of high VI change relatively little in viscosity as temperature increases, at least as compared to lubricants of lower VI. Since retention of viscosity at higher temperatures is a desirable characteristic, high viscosity index is desirable. Satisfactory viscosity properties may be conferred either by suitable choice of the lube base stock or by the use of VI improvers which are generally high molecular weight polymers. The extent to which VI properties can be varied by the use of these improvers is, however, limited because not only are large amounts of improver expensive but the improvers are subject to degradation in use so that service life of lubricants containing large amounts of improver may be limited. This implies that improvements in the VI of the base stock are desirable.
One approach to the problem of providing lube base stocks of satisfactory properties has been to employ synthetic base stocks. Base stocks of this type have typically been either synthetic esters similar to the ester type lubricants used in aviation gas turbines or, alternatively, synthetic hydrocarbon generally produced by the polymerization of low molecular weight olefins. Mixtures of the two have also been employed. (Examples of such lubricants are described in U.S. Pat. Nos. 3,997,621 (Oligomerization of 1-Olefins Over Boron Trifluoride Catalyst), U.S. Pat. Nos. 4,182,922 and 4,463,201 (Copolymers of Ethylene, Propylene and Other 1-Olefins) and U.S. Pat. Nos. 4,175,046 and 4,175,047 (Combinations of Synthetic Esters with Olefin Oligomers). Synthetic lubricants such as these have, however, been relatively expensive to produce and for this reason are generally sold in smaller quantities than the cheaper mineral oil lubricants.
Improvements have, of course, been made in the processing of mineral oil lubricants in order to improve the properties of the lube base stocks and a significant advance in this respect is described in European Patent Publication No. 225030 (Application No. 86308429.9 corresponding to U.S. Application Ser. No. 793,937, filed Nov. 1, 1985) which discloses a process for producing high VI lube base stocks from waxy refinery streams by a two stage process involving isomerization of waxy paraffins to isoparaffins of high VI, followed by a selective dewaxing step to obtain the desired pour point for the product. Lubricants produced by this process have extremely desirable properties of high VI coupled with low pour point but limitations on the volume production of this type of lubricant may be imposed by the availability of the waxy refinery streams which form the starting material for the process. The availability of these waxy feed stocks may be dependent upon the production of slack wax in refinery solvent dewaxing units and with the progressive replacement of solvent dewaxing by catalytic dewaxing, the availability of these feedstocks may become limited.
An alternative source of lubricants is provided by the Mobil olefins to gasoline/distillate/lubes process (MOGDL) which converts low molecular weight olefins to gasoline, distillate and lubricant fractions by oligomerization over an acidic, shape selective catalyst such as ZSM-5. This process employs as starting material the olefins which are available in large quantities from catalytic cracking units and is therefore capable of supplementing lubricant supplies to a considerable extent. However, the quality of the MOGDL lubricants is not as high as may be desired for severe service applications and accordingly it would be desirable to develop a way of improving the quality of these semi-synthetic lubricants.